1. Field of the Invention
This invention relates to integrated circuits (IC's) and particularly such circuits which have redundancy or spare capacity built into them.
2. Description of the Related Art
As integrated circuits become larger and more complex, the probability of conventional fabrication processes yielding a working circuit decreases. One method of overcoming this problem is the use of on-chip redundant circuit elements which can be selectively linked to the main circuit after the fabrication of the IC has been completed, to replace any defective elements in a large IC. In one known redundancy technique connections are broken by blowing metal fuses electrically. [Fitzgerald et al, "A 288Kb Dynamic RAM", Proceedings 1982 IEEE International Solid State Circuit Conference, page 68 (1982) and Ishihara et al, "A 256K Dynamic MOS RAM with Alpha Immune and Redundancy", ibid, page 74]. Another technique depends on blowing polysilicon fuses using a laser [Benevit et al, "256K Dynamic Random Access Memory", Proceedings 1982 IEEE International Solid State Circuit Conference, page 76 and Smith et al., "Laser Programmable Redundancy and Yield Improvement in a 64K DRAM", IEEE Journal of Solid-State Circuits, SC-16, p. 506, (1981)]. More recently, lasers have been used to make electrical connections. Vertical links are made between two metal layers separated by an insulating film when a laser drills a hole allowing metal to flow from one layer to another, [Raffel et al, "Laser Programmed Vias for Restructurable VLSI", IEDM Technical Digest, page 132 (1980)]. Lateral connections have also been made by laser welding two metal lines separated by a 2 micron gap to produce links of resistance under 1 ohm, [Yasaitas et al, "Low Resistance Laser Formed Lateral Links", IEEE Electron Device Letters, EDL-3, page 184 (1982)]. Links have also been formed with heavily doped polysilicon in which a short section remains intrinsic or high resistivity, [Minato et al, "A High-Speed Hi-CMOS II 4K Static RAM", IEEE Journal of Solid State Circuits 16, page 449, (1981)]. In this last method the application of a pulsed laser beam melts the intrinsic region and some of the neighboring doped polysilicon so that dopant flows into the intrinsic region and renders the link conductive. Links of resistance near 500 ohms have been formed.
Each of these three methods for forming lateral links has disadvantages. Sophisticated processing techniques are required for double metal levels, or for the fine lithography required for laser welding. These techniques can also be messy as molten metal may be splattered over the surrounding area. the methods require very precise positioning and focussing of the laser beam. Finally the polysilicon links do not have a low enough resistance for many applications.